Field of the Disclosure
The present invention relates generally to image sensors. More specifically, examples of the present invention are related to circuits that readout image data from image sensor pixel cells.
Background
Image sensors have become ubiquitous. They are widely used in digital cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (CMOS) image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of CMOS image sensors.
In a conventional CMOS active pixel sensor, image charge is transferred from a photosensitive device (e.g., a photo diode) and is converted to a voltage signal inside the pixel cell on a floating diffusion node. In conventional CMOS image sensors, an amplifier such as a source follower circuit is used in the pixel cells to amplify the signal on the floating diffusion node to output the image data to the bit lines, which are read by the column readout circuitry. Limited by the design and layout constraints of pixel cells, the source follower circuits can suffer from an unsatisfactory power supply rejection ratio, such as −20 dB. An unsatisfactory power supply rejection ratio can present many challenges, including noise from power supplies that can enter into the output signal path. The source follower circuits and bitline circuits can suffer from noise coupled form power supplies such as AVDD (VDD used in a pixel cell), HVDD (positive voltage pump), and NVDD (negative voltage pump). Furthermore, the coupling ratio from each power supply to readout signal path, and the ripple of power supplies can cause unwanted horizontal ripple in the captured images.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.